Stepping motors are available to cover a wide variety of applications and may range from quite expensive to inexpensive depending, in large measure, on the degree of structural complexity required of the motor to satisfy the needs of the particular application. The present invention deals with inexpensive motors which must approximate the accuracy of higher-priced stepping motors (e.g. accurate and repeatable step angles), but must be designed with economy and simplicity in mind so that they can be sold at relatively low cost. In the prior art, these type of motors are sometimes referred to as "tin-can" motors, because the motor housing is of a simple construction and in some respects closely resembles a tin can.
An example of such a motor is illustrated in U.S. Pat. No. 4,207,483 to Baer wherein a relatively simple construction for a stepping motor facilitates a relatively low selling price. The simple construction of the stepping motor is provided by a small number of parts which are not overly complex in design. Furthermore, the construction is without great tolerance demands and thereby enables relatively inexpensive manufacturing. Also, the assembly process is not overly complicated. All of the foregoing features combine to realize a motor that is at the same time relatively inexpensive and yet maintains reasonable performance characteristics.
In order to simplify the mechanical construction disclosed in the Baer patent, the motor requires each set of interdigital pole teeth to be formed of two annular stampings, one in contact with the metal shell housing and forming half the teeth, and the other in contact with an internal hub and forming the other half of the teeth. The teeth extend radially from a base portion of each stamping. On the stamping supported by the internal hub, the teeth extend radially outwardly. In a complementary manner, the teeth of the stamping supported by the housing extend radially inwardly and are circumferentially interleaved with the teeth extending radially outwardly. A conventional cylindrically wound coil is associated with the teeth such that when energized, the flux paths to the teeth traverse the housing and the internal hub. The magnetic poles thus formed by the teeth extend radially from their supporting and flux-carrying structures--i.e., the housing and the internal hub.
Although the foregoing structure provides an inexpensive stepping motor having performance characteristics suitable for many applications, the complex magnetic circuit that results from this structure leads to an imbalance in the magnetic paths which is dependent on the particular mode of energizing the coil.